ASHRAE Journal - April 2010 - 32

Standard 62.1 includes default values for occupant density. A note included under Table 6-1 clarifies that design engineers are not required to use this default, but may choose to if the actual occupant density is not known. Using an overly conservative estimate for zone population can result in significant overventilation and excessive energy use. It also impacts the calculation of the space latent cooling load, and the resulting dehumidification capacity of the dedicated OA unit. The recommendation of this author is to avoid applying safety factors to zone population (Pz), instead using the best estimate for expected occupancy during typical usage. Then, select the dedicated OA unit with “reserve” airflow capacity, as will be explained later in this article.
Safety Factor for Future Expansion or Change of Use

Cool + Reheat
OA C R C H CA

Active, Heat-Regenerated Desiccant
H OA Desiccant
Wheel Active

RG

C C CA

Cool + Reheat With Exhaust-Air Energy Recovery
OA EA EA
Total Energy Wheel C

Series Type III Desiccant Wheel With Exhaust-Air Energy Recovery
Series Desiccant Wheel Total Energy Wheel

R C H CA C C

OA EA

CA

EA

Figure 2: Example dedicated outdoor air equipment types.

The other common use of a safety factor when e determining the design airflow of the dedicated OA ur at er p unit occurs when accounting for future expansion m Te lb qL or a change in use of the facility. If the facility is -Bu t We expanded in the future, and the dedicated OA unit will be expected to serve the expansion, it would be prudent to select a unit with some amount of reserve (or extra) airflow capacity. Or, if the use of the facility is likely to change in the future (from an office space to a group of meeting rooms or a Dry-Bulb Temperature °F retail area, for example) the ventilation requirements for the zones served by the dedicated OA Figure 3: Dehumidification provided by a chilled water or direct-expansion dediunit may change. Again, it would be prudent to cated outdoor air unit. select a unit with reserve airflow capacity. The recommendation of this author is to calculate the design Since the previous section discussed the influence of safety airflow as accurately as possible, without using safety factors. factors on the design airflow (Vot), this section will focus on Then select a dedicated outdoor air unit that has reserve airflow the remaining two variables in Equation 3: the humidity ratio capacity, rather than a unit operating near its maximum allowable of the entering outdoor air (Woa) and the humidity ratio of the airflow. The impact of this will be discussed later in this article. leaving conditioned air (Wca).
Determining Design Dehumidification Capacity

The required dehumidification capacity of a dedicated OA unit (Figure 3, Equation 3) is dictated by the design airflow (Vot), humidity ratio of the entering outdoor air (Woa), and humidity ratio of the conditioned air leaving the unit (Wca): qL = 4.5 × Vot × (Woa – Wca) / 7000 gr/lb (3)

where qL = required dehumidification capacity, lb/h Vot = design outdoor airflow, cfm Woa= humidity ratio of the entering outdoor air, grains/ lb Wca= humidity ratio of the leaving conditioned air, grains/lb
32	 ASHRAE	Journal	

ASHRAE Handbook—Fundamentals7 is a popular source for climatic data that represents the outdoor design conditions for various locations. Historically, design engineers have used the design dry-bulb temperature and mean coincident wet-bulb temperature when calculating the required capacity of cooling systems. However, the highest outdoor humidity ratio does not occur at the same time as the highest outdoor dry-bulb temperature. Since 1997, the ASHRAE Handbook has included the design dew-point temperature and mean coincident drybulb temperature for use when calculating the required capacity of dehumidification systems. Table 1 lists the 0.4% outdoor design conditions for Jacksonville, Fla. 7 Notice that the outdoor humidity ratio is 32% higher at the design
ashrae.org	 	 April	 2010

Safety Factor for Woa (Humidity Ratio of the Entering Outdoor Air)

Humidity Ratio, grains/lb of Dry Air

°F



ASHRAE Journal - April 2010

Table of Contents for the Digital Edition of ASHRAE Journal - April 2010

ASHRAE Journal - April 2010
Contents
Commentary
Industry News
Letters
Meetings and Shows
Feature Articles
The Science of Evaporation is Key to Defense in Murder Trial
Selecting DOAS Equipment with Reserve Capacity
Technology Award Case Studies: Greening Hospitals
Technology Award Case Studies: Sustainable Remedy for Hospital
Building Sciences
Emerging Technologies
Technical Topics: Selecting Efficient Fans
Technical Topics: Dual-Capacity Heat Pumps
IAQ Applications
International Column
Classified Advertising
Advertisers Index
ASHRAE Journal - April 2010 - Intro
ASHRAE Journal - April 2010 - ASHRAE Journal - April 2010
ASHRAE Journal - April 2010 - Cover2
ASHRAE Journal - April 2010 - 1
ASHRAE Journal - April 2010 - 2
ASHRAE Journal - April 2010 - Contents
ASHRAE Journal - April 2010 - Commentary
ASHRAE Journal - April 2010 - 5
ASHRAE Journal - April 2010 - Industry News
ASHRAE Journal - April 2010 - 7
ASHRAE Journal - April 2010 - 8
ASHRAE Journal - April 2010 - 9
ASHRAE Journal - April 2010 - 10
ASHRAE Journal - April 2010 - 11
ASHRAE Journal - April 2010 - 12
ASHRAE Journal - April 2010 - 13
ASHRAE Journal - April 2010 - 14
ASHRAE Journal - April 2010 - 15
ASHRAE Journal - April 2010 - 16
ASHRAE Journal - April 2010 - 17
ASHRAE Journal - April 2010 - Letters
ASHRAE Journal - April 2010 - 19
ASHRAE Journal - April 2010 - Meetings and Shows
ASHRAE Journal - April 2010 - 21
ASHRAE Journal - April 2010 - The Science of Evaporation is Key to Defense in Murder Trial
ASHRAE Journal - April 2010 - 23
ASHRAE Journal - April 2010 - 24
ASHRAE Journal - April 2010 - 25
ASHRAE Journal - April 2010 - 26
ASHRAE Journal - April 2010 - 27
ASHRAE Journal - April 2010 - 28
ASHRAE Journal - April 2010 - 29
ASHRAE Journal - April 2010 - Selecting DOAS Equipment with Reserve Capacity
ASHRAE Journal - April 2010 - 31
ASHRAE Journal - April 2010 - 32
ASHRAE Journal - April 2010 - BRC1
ASHRAE Journal - April 2010 - BRC2
ASHRAE Journal - April 2010 - 33
ASHRAE Journal - April 2010 - 34
ASHRAE Journal - April 2010 - 35
ASHRAE Journal - April 2010 - 36
ASHRAE Journal - April 2010 - 37
ASHRAE Journal - April 2010 - 38
ASHRAE Journal - April 2010 - 39
ASHRAE Journal - April 2010 - 40
ASHRAE Journal - April 2010 - 41
ASHRAE Journal - April 2010 - Technology Award Case Studies: Greening Hospitals
ASHRAE Journal - April 2010 - 43
ASHRAE Journal - April 2010 - 44
ASHRAE Journal - April 2010 - 45
ASHRAE Journal - April 2010 - 46
ASHRAE Journal - April 2010 - 47
ASHRAE Journal - April 2010 - 48
ASHRAE Journal - April 2010 - 49
ASHRAE Journal - April 2010 - Technology Award Case Studies: Sustainable Remedy for Hospital
ASHRAE Journal - April 2010 - 51
ASHRAE Journal - April 2010 - 52
ASHRAE Journal - April 2010 - 53
ASHRAE Journal - April 2010 - Building Sciences
ASHRAE Journal - April 2010 - 55
ASHRAE Journal - April 2010 - 56
ASHRAE Journal - April 2010 - AP1
ASHRAE Journal - April 2010 - AP2
ASHRAE Journal - April 2010 - AP3
ASHRAE Journal - April 2010 - AP4
ASHRAE Journal - April 2010 - 57
ASHRAE Journal - April 2010 - 58
ASHRAE Journal - April 2010 - 59
ASHRAE Journal - April 2010 - Emerging Technologies
ASHRAE Journal - April 2010 - 61
ASHRAE Journal - April 2010 - 62
ASHRAE Journal - April 2010 - 63
ASHRAE Journal - April 2010 - Technical Topics: Selecting Efficient Fans
ASHRAE Journal - April 2010 - 65
ASHRAE Journal - April 2010 - Technical Topics: Dual-Capacity Heat Pumps
ASHRAE Journal - April 2010 - 67
ASHRAE Journal - April 2010 - 68
ASHRAE Journal - April 2010 - 69
ASHRAE Journal - April 2010 - IAQ Applications
ASHRAE Journal - April 2010 - 71
ASHRAE Journal - April 2010 - 72
ASHRAE Journal - April 2010 - 73
ASHRAE Journal - April 2010 - International Column
ASHRAE Journal - April 2010 - 75
ASHRAE Journal - April 2010 - 76
ASHRAE Journal - April 2010 - 77
ASHRAE Journal - April 2010 - 78
ASHRAE Journal - April 2010 - 79
ASHRAE Journal - April 2010 - 80
ASHRAE Journal - April 2010 - 81
ASHRAE Journal - April 2010 - 82
ASHRAE Journal - April 2010 - 83
ASHRAE Journal - April 2010 - 84
ASHRAE Journal - April 2010 - 85
ASHRAE Journal - April 2010 - Classified Advertising
ASHRAE Journal - April 2010 - 87
ASHRAE Journal - April 2010 - Advertisers Index
ASHRAE Journal - April 2010 - Cover3
ASHRAE Journal - April 2010 - Cover4
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